The crustal architecture of the central Skellefte district, Sweden Structural analysis, setting of VMS deposits and 3D-modelling
Sammanfattning: The Skellefte district in northern Sweden is a Palaeoproterozoic volcanic arc hosting abundant volcanogenic massive sulphide (VMS) deposits. The dominating rocks in the district are submarine volcanic and volcaniclastic rocks of the Skellefte Group deposited in an extensional regime at 1.89–1.88 Ga. Structural analysis and facies analysis combined with reflection seismic investigations in the central part of the Skellefte district revealed a pronounced pattern of WNW–ESE-striking normal faults and NE–SW-striking transfer faults that developed synchronously with volcanic activity. Dextral strike-slip movement along regional-scale NE-SW-striking faults favoured the opening of a pull-apart system. VMS deposits formed as sub-seafloor replacement and partly exhalative deposits mainly in the uppermost parts of Skellefte Group but also in the lowermost parts of the overlying Vargfors Group. Furthermore, the VMS deposits show a close spatial relationship to faults indicating that the ore-forming hydrothermal fluids utilized the synextensional faults as fluid conduits. The Vargfors Group sedimentary rocks form a distinct sub-basin in the central Skellefte district, the so called Vargfors basin or Vargfors syncline. Syn-extensional faulting created fault-bound compartments within the Vargfors basin. Syn-tectonic sedimentation of the Vargfors Group within the sub-basin is responsible for changes in stratigraphy between the individual compartments. Progressive opening of the sub-basin from the centre towards north-west and south-east is evident from the change of sedimentary facies associations. The onset of Vargfors Group deposition is characterized by turbiditic sedimentation with intercalated sandstones and monomict conglomerates derived from eroded and/or reworked Skellefte Group volcanic rocks. These facies associations are unconformably overlain by polymict conglomerates of alluvial fan and braided systems caused by the uplift of the Jörn intrusive complex and the subsequent formation of the Arvidsjaur volcanic arc, respectively. It is unclear if the polymict series formed during the latest stages of crustal extension or at the onset of subsequent crustal shortening. Crustal shortening at around 1.87 Ga is attributed to basin inversion and accretionary processes at the Svecokarelian craton margin and is assumed to result in inversion of normal and transfer faults and the formation of upright, fault-bound syn- and anticlines. A rheologically weak carbonate-rich layer at the base of the sedimentary sequence is suggested to favour the fault inversion over more distributed shortening as the controlling deformation mechanism in the Vargfors syncline. Furthermore, VMS deposits were transposed along the faults, and the deformation style of the ore bodies generally mimics the deformation styles of the hosting high-strain zones. Moreover, the size of the VMS deposits is coupled to the size of the associated high-strain zone with large-tonnage deposits associated with regional-scale high-strain zones and their splays. Progressive crustal shortening led to the formation of N-dipping break-back and shortcut faults. Mafic volcanic rocks are commonly observed along early, inverted faults and later break-back and short-cut faults indicating a long history of mafic volcanic activity both during extension and crustal shortening in the central Skellefte district. Three-dimensional geological multi-scale modelling combined results from geological and geophysical investigations in order to visualise the crustal architecture in the Skellefte district. Modelling was carried out in different scales ranging from detailed deposit-scale models of 21 VMS ore bodies to a semi-regional scale model of the Vargfors syncline and a simplified regional scale model of the central Skellefte district.
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